Famfamfam_flags is a project mainly written in ..., it's free.
The flag icon set of famfamfam.com
This is a copy of the famfamfam flag icons, created by Mark James, Birmingham, UK. It is intented to be used as git submodule.
There are some projects using the famfamfam flags icon set. Most developers simply put a copy of the flags icon set into their repository, wasting space. Using the flag icons as git submodule is more elegant and keeps your main repository pollution-free. As a bonus, you can fork and commit your own icons - and eventually merge them back.
The flag filenames correspond to the [ISO 3166-1 alpha-2 country codes]http://en.wikipedia.org/wiki/ISO_3166-1_alpha-2
These icons are licensed as public domain; you may use them freely for any purpose with no requirement for attribution.
The author's homepage can be found at http://www.famfamfam.com/lab/icons/flags/